The particle size of a latex can often have a direct impact on the performance of a coating prepared from that latex. Among those properties that can be affected are gloss, clarity, film formation, and substrate penetration (i.e., for porous substrates). In a very general sense, a smaller particle size will have a positive effect on such properties. Small particle sizes (i.e., &lt;100 nm) are most often achieved by using relatively high levels (2-4 wt) of small molecule, typically anionic, surfactants such as AEROSOL-OT, sodium dioctyl sulfosuccinate and AEROSOL NPES 2030, ammonium nonylphenoxy polyethoxy ethanol sulfate, which are anionic surfactants sold by Cytec Industries, Inc. However, small particle size and the properties directly affected by it are not the only important properties in water-based coatings. Of primary importance is the water-resistance/sensitivity of the final film. In small particle size systems, the high level of anionic surfactant, the very component in the latex which gives the small particle size and all of its concommitant advantages, is also likely to be detrimental to water-resistance.
One further challenge in the preparation of any stable latex is production of a stable emulsion with minimal amounts of coagulum. As noted in Emulsion Polymers and Emulsion Polymerization, "The Formation of Coagulum in Emulsion Polymerization", by J. W. Vanderhoff, 1981, American Chemical Society, coagulum, i.e., polymer recovered in a form other than that of a stable latex, is produced in all sizes of reactors and poses several problems. Such problems include loss of yield of the desired latex, processing difficulties due to the necessity of clean-up, more batch-to-batch variation in latex properties, and health, safety, and environmental problems insofar as the coagulum must be disposed of; this is made more problematic due to entrapment within the coagulum of toxic monomers such as vinyl chloride and acrylonitrile. According to this reference, the formation of coagulum is due to either a failure of the colloidal stability of the latex during or after polymerization, which leads to flocculation of the particles or by polymerization of the monomer(s) by mechanisms other than by the intended emulsion polymerization.
U.S. Pat. No. 5,342,877 describes a method for preparing small particle size latexes via copolymerization of hydroxyalkyl (meth)acrylates (15-40 weight percent based on total latex solids) and other vinyl/acrylic monomers (particularly styrene) in the presence of water-dispersible polyesters.
U.S. Pat. No. 4,939,233 describes a method for preparing water-dispersible polyester/vinyl acetate copolymer blends via emulsion polymerization using sulfonated polyesters as stabilizers in the reaction.
U.S. Pat. Nos. 4,946,932 and 5,277,978 describe a methods for preparing water dispersible polyester/-acrylic copolymer blends via emulsion polymerization in the presence of sulfonated polyesters as stabilizers.
U.S. Pat. No. 5,156,651 describes water dispersible polyester/vinyl aromatic latexes for textile sizing applications.
U.S. Pat. No. 4,839,413 describes the use of low molecular weight (i.e., less than 20,000) alkali-soluble resins as "support resins" in emulsion polymerization. The support resin is formed via non-aqueous polymerization methods and is subsequently dispersed/dissolved in alkaline solution. The emulsion polymerization is then carried out, at high pH, in the presence of this dissolved support resin and an additional costabilizer (surfactant). A pH of greater than 8 is taught to be necessary.
U.S. Pat. No. 5,326,843 describes a process for preparing low molecular weight (i.e., less than 40,000) alkali-soluble polymers via emulsion polymerization. Unsaturated aromatic monomers (e.g. styrene), methacrylic acid, and low pH (less than 4.5) are specifically taught.
U.S. Pat. No. 4,325,856 describes the preparation of a multistage latex in which the first stage is more hydrophilic (via incorporation of acid functional monomers) than the second stage thereby resulting in an inverted core-shell morphology. Typical anionic surfactants such as potassium n-dodecyl sulfate, sodium isooctylbenzene sulfonate, sodium laurate, and nonylphenol esters of polyethylene glycols are used to stabilize the latexes. Latex particle sizes of 130-160 nm are reported. Unlike the methodology taught in U.S. Pat. No. 4,839,413, complete neutralization of the acid-functional first stage is not necessary.
U.S. Pat. No. 4,150,005 describes the preparation of a multistage latex in which the first stage is more hydrophilic than the second stage, thereby resulting in an inverted core-shell morphology. Typical small molecule anionic surfactants such as alkali metal and ammonium salts of alkyl, aryl, alkaryl, and aralkyl sulfonates, sulfates and polyether sulfates, and corresponding phosphates and phosphonates, and ethoxylated fatty acids, esters, alcohols, amines, amides and alkyl phenols, are used to stabilize the latexes. As with the methodology described in U.S. Pat. No. 4,325,856, complete neutralization of the first stage is not necessary. Furthermore, combinations of methacrylic acid and hydroxyethyl methacrylate as hydrophilic components of the first stage are claimed as is a range of hydrophilic copolymer of 20-80 wt % of the total latex. The advantages of using the hydroplasticized first stage to lower minimum filming temperatures are also described.
U.S. Pat. No. 4,916,171 describes technology very similar to that described in U.S. 4,150,005 with the exception that the only hydrophilic monomers claimed are carboxylic acids. A much broader range for the hydrophilic polymer of 1-99 wt % of the total latex is claimed. This reference also teaches that the hydrophilic shell polymer has a very high Tg (&gt;100.degree. C.) and that the most preferred range of hydrophilic polymer is 40-60 wt % of the total latex.
J. Appl. Polymer Sci., 44, 1075 (1992) (M. Lambla et al.) describes the terpolymerization of styrenelbutyl acrylate/methacrylic acid in the presence of low levels of sodium dodecyl sulfate, a small molecule surfactant, with the subsequent copolymerization of styrene/butyl acrylate in the presence of the acid functional latex. Inverted core-shell latexes (i.e. acid functional 1st stage at particle surface) with average particle sizes of 250 nm resulted from this method. As with U.S. Pat. No. 4,839,413 described above, high pH is a critical part of the process for preparing these materials.